1. Field of the Invention
This invention relates generally to communications within a computer system. More specifically, it relates to a fiber optic interface between a main processor and a peripheral processor.
2. Background Information
Many computer systems contain multiple peripheral devices in order to increase their processing power. The main processors of the computer system must communicate with the peripherals in an efficient and reliable manner. In a large computer system such as the 2200/900 computer, available from Unisys Corporation, there may be configured many mass storage devices. The Input/Output (I/O) channels between the main processors and these peripherals are physically composed of cables containing hard-wired conductors. Access time to the peripherals over the cables is a critical issue when analyzing the overall performance of the computer system. As processor performance has increased, the bandwidth and throughput of wire cables has become a bottleneck for system performance. The advent of fiber optic communications technology promises an attractive solution to the I/O bandwidth and throughput problem.
The use of fiber optics as an interconnection medium between a main processor and peripheral processors provides a number of benefits. Most importantly, it provides substantially higher data transfer rates and longer transmission distances compared with wire buses traditionally used for I/O interfaces. In addition, fiber optic interfaces have very high noise immunity and low error rates. Cable bulk is also reduced and connector reliability is enhanced. Recently the cost of fiber optic technology has been reduced to the point where the use of reliable, efficient fiber optic links between processors is a good overall design choice for an I/O interface. But the use of cumbersome standards such as the Fiber Distributed Data Interface (FDDI) standard may not satisfy requirements for minimizing protocol overhead. The FDDI standard, which is primarily used for token ring local area networks (LANs), is overly complex and subject to change only by committee. In addition, the FDDI token ring protocol operates effectively only in a half-duplex mode. A more streamlined, flexible approach with increased throughput for point-to-point fiber optic communications is needed.
There are prior art systems that use fiber optic interfaces. In Cecchini, U.S. Pat. No. 5,150,244 there is disclosed an interface for connecting two independent half-duplex networks. This interface is essentially a fiber optic bridge between two local-area networks. This prior art system is deficient in that it only provides half-duplex communications and only interconnects separate networks rather than directly coupling a main processor and a peripheral processor. As such it has limited applicability to a variety of device interconnection problems.
The July 1992 issue of the IBM Journal of Research and Development describes the Enterprise Systems Connection (ESCON) architecture used to connect processors and peripherals in the IBM System/390 family of computer systems. The ESCON architecture uses fiber optic serial-I/O channels to pass messages between devices in the system. The ESCON architecture employs a dynamic crosspoint switch as an interconnection topology rather than a dedicated point-to-point topology as is needed for a single main processor to peripheral processor coupling. Thus, ESCON uses extra processing time to perform path and message switching, which is unnecessary in a dedicated point-to-point application. Interoperability is also a concern with the ESCON architecture. Although it implements the FDDI standard, it does not support cascaded operations to transfer multiple bytes of data packets in parallel. Finally, the ESCON architecture is designed to work in a network environment so its complex protocol is not streamlined to provide maximum efficiency in a dedicated point-to-point application.